Voltage references are commonly used in the electronics industry to provide known voltages to systems and circuits. The use of such references allows the design and manufacture of stable supply voltages and the monitoring and measuring of events. It is desirable for a voltage reference to be stable across temperature. A bandgap voltage reference generator is a known type of voltage reference generator, which is stable across temperature. The bandgap voltage reference generator operates by summing together a junction voltage Vj and a voltage that is proportional to absolute temperature (Vpat). FIG. 1 illustrates a known circuit 100 used to generate the voltage Vj 102. The circuit 100 has a current source 104 connected to a diode 106, such that the diode 106 is forward biased. The voltage Vj is the forward bias voltage of the diode 106. FIG. 2 illustrates graphically the known transfer characteristic curve of the voltage Vj 102 over temperature. The horizontal-axis of the graph represents absolute temperature in degrees Kelvin. The vertical-axis represents the voltage Vj 102. At room temperature, a junction formed with a typical process would have a junction voltage Vj 102 between 0.4 and 0.7 volts. At 0.degree. K., the junction voltage would be limited by the bandgap voltage (V.sub.BG). The bandgap voltage for silicon is a known, nearly constant value of approximately 1.2 volts. Between 0.degree. K. and room temperature, 300.degree. K., the transfer characteristic curve is represented by a nearly linear curve between the voltage value at 0.degree. K. and the voltage value at 300.degree. K. These voltage values are approximately 1.2 volts and 0.5 volts respectively. Therefore, the transfer characteristic curve has a negative slope.
FIG. 3 illustrates a known voltage proportional to temperature (VPT) generator circuit 300 used to generate the voltage Vpat. While a VPT generator can be generated in either MOS or bipolar technology, the illustrated VPT generator 300 depends on the exponential diffusion dominant nature of the sub-threshold current in an MOS device. As such, if the current densities in transistors 308 and 306 are sufficiently small so that the transistors operate in the sub-threshold region, also called the weak inversion region, a voltage proportional to absolute temperature will be present at node 312, providing the width of transistor 306 is greater than that of transistor 308, their lengths are substantially identical, and the gate electrode voltage applied to both transistors 306 and 308 is such that they both carry the same value of current. FIG. 4 illustrates a known transfer characteristic curve for the VPT generator 300. The vertical-axis represents the voltage proportional to absolute temperature (Vpat); the horizontal-axis represents the temperature in degrees Kelvin. As the temperature approaches absolute zero (0.degree. K.), Vpat approaches zero volts. The transfer characteristic curve is represented by a line between a value of 20 to 80 millivolts at room temperature, depending on the ratio of the size of transistor 306 to the size of transistor 308 and the particular device process technology used for the transistors, and zero volts at absolute zero. The curve is linear and has a positive slope.
FIG. 5 illustrates a transfer characteristic curve for a Vj labeled 504 and an amplified transfer characteristic curve for a Vpat labeled 506. The transfer characteristic curve for Vj 202 (FIG. 2), and the transfer characteristic curve for Vpat (FIG. 4), have slopes in opposite directions. Amplifying the slope of Vpat (FIG. 4) provides a slope equal to but opposite that of the slope of Vj 505. This amplified slope is represented by the curve 506. Adding the Vj voltage curve 504 and the amplified voltage curve 506 provides a voltage reference curve (Vref) 502 that is independent of temperature variation. The slope of Vref curve 502 is essentially zero. The further use of voltage shifting techniques allows the voltage reference level to be shifted to values above or below 1.2 volts.
While the use of bandgap voltage reference generators is widespread, they have been limited to use with power supply voltages above the bandgap voltage of approximately 1.2 volts. Present applications requiring batteries and lower voltages have created a need for voltage references below the 1.2 volts reference value. Therefore, the need exists for a bandgap type voltage reference generator that can operate with and generate low voltages, and for a voltage reference generator that can generate multiple reference voltages.